occurs in shallow water lagoonal environments. ... The Wadi As Shati area is mostly desert. It ... important iron ore bearing cycles of sedimentation in the Wadi ... SGA 2017-14th Biennial Meeting of Society for Geology Applied to Mineral Deposit, Québec, Canada .... chert bands, the SiO2 being mainly present in iron silicate.
Proceeding Book Geochemistry of Iron Ore at Wadi As Shati, SW Libya: Implications on Origin, Depositional Environment, Paleooxygenation, Paleoclimate and Age Osama R. Shaltami1, Patrizia Fiannacca2, Fares F. Fares1, Farag M. EL Oshebi1, George D. Siasia3 and Hwedi Errishi4 1Department 2 Department
of Earth Science, Faculty of Science, Benghazi University, Libya
of Biological, Geological and Environmental Sciences, Faculty of Science, University of Catania, Italy 3International 4Department
Braking and Railway Equipment (IBRE), France
of Geography, Faculty of Arts, Benghazi University, Libya
SGA 2017-14th Biennial Meeting of Society for Geology Applied to Mineral Deposit, Québec, Canada
Abstract
Introduction The oolitic ironstone bed represents the upper parts
Wadi As Shati is situated within the Murzuq Basin of
of both Dabdab and Tarut formations, while it is characteristic
southwest Libya. The Wadi As Shati area is mostly desert. It
of the middle part of Ashkidah Formation. The detected iron
borders on Nalut in northwest, Jabal Al Nafusah in the north, Al
minerals are goethite, siderite, hematite, limonite, magnetite,
Jufrah in the east, Sabha in the southeast, Wadi Al Haya in the
chamosite and pyrite. Generally, oolitic ironstone deposition
south, Ghat in the southwest and Illizi Province of Algeria in the
occurs in shallow water lagoonal environments. Fe2O3
west. The study area is the eastern area of a part of the Wadi
represents between 43.57 to 64% of all ironstones contents.
As Shati, between longitudes 13o 45’ 00” and 14o 15’ 00” E and
The discrimination diagrams point to a hydrogenous source for
latitudes 27o 20’ 00” and 27o 40’ 19’’ N (Fig. 1). The three
Fe mineralization. Climatic conditions of semi-humid to semi-
formations; Dabdab, Tarut and Ashkidah, represent the most
arid prevailed during the deposition of the ironstones. The
important iron ore bearing cycles of sedimentation in the Wadi
molybdenite ages for Dabdab, Tarut and Ashkidah formations
As Shati area. The distribution of the three formations in the
are 379.2 ± 1.1 Ma (Fransnian, Late Devonian), 362.5 ± 1.1
study area is shown in Fig (2). The oolitic ironstone bed
Ma (Famennian, Late Devonian) and 355.6 ± 1.1 Ma
represents the upper parts of both Dabdab and Tarut
(Tournaisian, Early Carboniferous), respectively.
formations, while it is characteristic of the middle part of Ashkidah Formation (Fig. 3). This bed was originally named
Keywords: Geochemistry, Ironstones, Origin, Depositional
the ore bearing member “L” in the Dabdab Formation, ore
Environment, Paleooxygenation, Paleoclimate, Age Dating,
bearing member “A” in the Tarut Formation and ore bearing
Wadi As Shati, Murzuq Basin, Libya.
member “B” in the Ashkidah Formation (Seidl and Rohlich, 1984).
Fig. 1: Location map of the study area
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SGA 2017-14th Biennial Meeting of Society for Geology Applied to Mineral Deposit, Québec, Canada
Fig. 2: Geological map of the study area showing the distribution of Dabdab, Tarut and Ashkidah formations (modified after Seidl and Rohlich, 1984)
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SGA 2017-14th Biennial Meeting of Society for Geology Applied to Mineral Deposit, Québec, Canada
Fig. 3: Composite columnar section of Dabdab, Tarut and Ashkidah formations in the study area The present work attempts to characterize the mineral
not enough. The age and origin of these iron ores has been
and chemical compositions of the iron ores at Wadi As Shati,
matter of debate.
SW Libya, with especial emphases on origin, depositional environment, paleooxygenation, paleoclimate and age dating
Methodology
of these sediments. As far as the authors are aware, the
Eighteen representative samples of
published data on the oolitic ironstones in the study area are
the oolitic
ironstones were collected from Dabdab, Tarut and Ashkidah
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SGA 2017-14th Biennial Meeting of Society for Geology Applied to Mineral Deposit, Québec, Canada
formations (six samples of each formation, see Fig. 3). All the
contain high amount of MnO (3.45%, in average), while the
samples were prepared as polished thin sections. The thin
MnO content is relatively low in the samples of Tarut and
section preparation was done in the Thin Section Lab, Toul,
Ashkidah formations (0.69, in average). Oolitic ironstones are
France. The mineral composition of the studied samples was
recognized as being enriched in many trace elements such as
determined by using petrographic study under transmitted
V, Ba, Sr, Co, Zr, Y, Ni, Zn, and Cu (Tobia et al., 2014). In
polarizing and reflected light microscopes.
addition, anomalous P, V, Cr, Ni, Zn, As, Mo, and U are commonly correlated with Fe-oxyhydroxides (Salama et al.,
The major oxide contents were determined by atomic
2012). The studied samples contain high concentrations of Ni,
absorption spectroscopy. Loss on ignition (LOI) was measured
Co, V, Cr, Mo, Pb, As Zr, Nb, Th, U and REE. The REE are
from the total weight after ignition at 1000°C for 2 h. Trace
normalized to Post-Archean Australian Shale (PAAS, Taylor
element contents were determined by inductively coupled
and McLennan, 1985). PAAS-normalized REE patterns of the
plasma-mass
These
studied samples show distinctive positive Ce- and Eu-
analyses were done in the ACME analytical laboratories of
anomalies and enrichment in the HREE over the LREE (Fig.
Vancouver, Canada. The Re and Os concentrations of three
5).
spectrometry
(ICP-MS)
technique.
molybdenite samples were determined by negative thermal ionization mass spectrometry (N-TIMS) using a Finnigan MAT-
Classification of Oolitic Ironstones
262 at the Institut de Physique du Globe de Paris, France.
Oolitic ironstones are sedimentary rocks with >5% ooids and >15% iron, corresponding to 21.4% Fe2O3 (Petranek
Results and Discussions
and Van Houten, 1997; Mucke and Farshad, 2005). Oolitic
Mineral Composition
ironstones accumulated throughout the Phanerozoic Eon, from
On the basis of mineral composition, the studied
the Cambrian to the Recent (Petranek and Van Houten, 1997),
oolitic ironstone beds can be divided into two facies:
following the earlier Precambrian banded iron formations. More than 500 deposits are known, summarized as Phanerozoic
1) Reduced Facies (Lower Facies): In the Dabdab Formation,
oolitic ironstones. They were especially common in the
this facies contains pyrite, chamosite and siderite (Fig. 4),
Ordovician and the latter part of the Silurian, Devonian and
while the reduced iron minerals in the Tarut and Ashkidah
again in the Jurassic and Cretaceous. In contrast, only a few
formations are magnetite, chamosite (see Fig. 4) and siderite.
occurred in the Cambrian, Permian, Triassic, or in the Upper
The reduced facies in all formations contains small amounts of
Cenozoic (Petranek and Van Houten, 1997).
quartz, chlorite, molybdenite, apatite, graphite, zircon and rutile (see Fig. 4).
In the Mediterranean countries, oolitic ironstone deposits of Paleozoic age are documented in France, Algeria
2) Oxidized Facies (Upper Facies): In the Dabdab Formation,
and Libya (Chauvel and Guerrak, 1989). James (1992) showed
the oxidized facies contains limonite and goethite (see Fig. 4)
that Phanerozoic ironstones can be distinguished from
with lesser amounts of gypsum, quartz, kaolinite and
Precambrian iron formations by using the SiO2–FeO+MgO–
manganese oxides (bixbyite, jacobsite and cryptomelane, see
Fe2O3 diagram (Fig. 6). According to Evans (1993) the
Fig. 4), while in the Tarut and Ashkidah formations, this facies
Phanerozoic ironstones can be classified into two types:
contains hematite and limonite (see Fig. 4) and small amounts of quartz and kaolinite.
1) Clinton Type: This forms massive beds of oolitic hematitechamosite-siderite rock. The Fe2O3 content is about 40-50%
Geochemistry
and they are higher in Al2O3 and P2O5 than banded iron
The concentrations of major oxides and trace
formation (B.I.F.). They also differ from B.I.F. in the absence of
elements of the studied ironstones are given in tables (1-2).
chert bands, the SiO2 being mainly present in iron silicate
Fe2O3, SiO2 and Al2O3 represent between 76.72 to 86.30% of
minerals with small amounts as clastic quartz grains. The
all ironstones contents. The Dabdab Formation samples
Clinton type is common in rocks of Cambrian to Devonian age.
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SGA 2017-14th Biennial Meeting of Society for Geology Applied to Mineral Deposit, Québec, Canada
Fig. 4: Photomicrographs of (a) goethite (sample D5), (b) siderite (sample D1), (c) hematite (sample T4), (d) limonite (sample S6), (e) magnetite (sample T2), (f) chamosite (sample S1), (g) pyrite (sample D3) (h) quartz (sample T4), (i) zircon (sample S3), (j) apatite (sample S1), (k) rutile (sample T2), (l) kaolinite (sample D5), (m) molybdenite (sample T3), (n) graphite (sample S1), (o) gypsum (sample D6), (p) chlorite (sample T3), (q) bixbyite (sample D4) and (r) jacobsite altered to cryptomelane (sample D6)
Table 1: Major oxide concentrations (wt%) of the oolitic ironstones Formation Member Facies Sample No. SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O SO3 Cl P2O5 LOI Total
Dabdab L Reduced
Tarut A Oxidized
Reduced
D1 D2 D3 D4 D5 D6 T1 T2 17.29 17.00 17.45 19.00 19.19 18.88 22.87 22.60 0.61 0.53 0.66 0.69 0.70 0.67 0.78 0.73 7.33 7.12 7.53 9.87 9.94 9.58 14.75 14.53 61.68 62.08 61.11 54.44 54.21 54.70 44.40 45.00 3.83 4.00 3.67 3.08 3.00 3.13 0.46 0.51 0.22 0.25 0.26 0.18 0.20 0.20 1.09 1.11 0.51 0.55 0.57 1.44 1.39 1.37 1.05 0.97 0.10 0.07 0.14 0.19 0.21 0.17 2.17 2.00 0.19 0.14 0.21 0.41 0.44 0.39 1.11 1.05 0.20 0.31 0.18 0.69 0.55 0.71 0.06 0.10 0.13 0.16 0.15 0.08 0.08 0.08 0.92 0.90 0.88 0.93 0.95 1.22 1.13 1.19 0.22 0.28 7.00 6.82 7.07 8.66 8.92 8.88 10.08 10.17 99.97 99.96 99.95 99.95 99.96 99.95 99.96 99.95
Ashkidah B Oxidized
T3 22.75 0.75 14.69 44.72 0.48 1.15 1.11 2.05 1.09 0.09 0.88 0.29 9.91 99.96
Page 41
T4 16.89 0.50 7.09 52.84 1.18 0.72 2.55 0.08 0.11 0.96 0.23 1.80 15.00 99.95
T5 16.68 0.48 6.93 53.11 1.21 0.87 2.41 0.08 0.10 0.98 0.24 1.91 14.96 99.96
Reduced T6 16.50 0.46 6.81 53.42 1.25 0.89 2.47 0.08 0.09 0.98 0.22 1.90 14.88 99.95
S1 10.45 0.23 5.24 63.67 0.62 0.28 2.44 0.06 0.07 4.21 0.19 0.77 11.73 99.96
S2 10.91 0.25 5.55 63.22 0.53 0.30 2.39 0.06 0.08 4.06 0.27 0.80 11.53 99.95
Oxidized S3 10.31 0.21 5.13 64.00 0.69 0.33 2.47 0.06 0.07 4.27 0.20 0.83 11.39 99.96
S4 23.21 1.33 10.11 43.57 0.40 0.14 0.10 2.37 1.23 0.14 0.09 1.49 15.78 99.96
S5 23.09 1.25 10.05 43.80 0.44 0.17 0.11 2.29 1.20 0.17 0.10 1.33 15.97 99.97
S6 23.00 1.19 9.98 43.91 0.49 0.20 0.11 2.24 1.18 0.19 0.10 1.37 16.00 99.96
SGA 2017-14th Biennial Meeting of Society for Geology Applied to Mineral Deposit, Québec, Canada
Table 2: Trace element concentrations (ppm) of the oolitic ironstones Dabdab L
Tarut A
Reduced D1 126.28 24.71 864.38 176.08 69.29 23.28 19.96 22.84 35.97 169.97 2.46 115.14 1.12 25.46 36.28 5.32 68.29 26.05 206.11 14.27 79.08 18.73 4.63 18.49 3.29 17.81 4.28 11.78 1.69 11.15 1.79
D2 126.68 25.11 864.78 176.48 62.69 23.68 20.36 23.24 36.37 220.88 2.37 166.45 1.03 26.86 36.68 6.72 68.38 26.14 206.20 14.36 79.17 18.82 4.72 18.58 3.38 17.90 4.37 11.87 1.78 11.24 1.88
Oxidized D3 125.71 24.14 863.81 175.51 61.72 22.71 19.39 22.27 35.40 243.74 1.23 188.91 0.89 26.89 35.71 6.75 68.20 25.96 206.00 14.18 78.99 18.64 4.54 18.40 3.20 17.72 4.19 11.69 1.60 11.06 1.70
D4 194.04 192.47 557.14 295.84 15.11 91.04 12.78 27.66 28.79 121.21 1.70 66.38 0.75 111.72 24.05 11.58 76.55 32.86 211.89 16.43 78.21 18.65 4.59 19.88 3.40 19.00 4.34 12.64 1.75 11.63 1.88
D5 193.81 192.24 556.91 298.61 14.82 90.81 12.49 25.37 28.50 186.88 1.37 132.25 1.03 111.49 23.81 11.35 76.62 32.93 211.96 16.50 78.28 18.72 4.66 19.95 3.47 19.07 4.41 12.71 1.82 11.70 1.95
Ashkidah B
Reduced D6 194.30 192.73 557.40 299.10 15.31 91.30 12.98 29.86 28.99 164.32 0.81 109.49 0.47 111.98 24.30 11.84 76.48 32.79 211.82 16.36 78.14 18.58 4.52 19.81 3.33 18.93 4.27 12.57 1.68 11.56 1.81
T1 114.60 22.13 957.00 162.45 35.00 11.60 2.67 25.55 18.68 80.67 1.16 25.84 0.45 22.88 59.60 2.74 77.47 37.14 226.48 17.40 81.81 18.50 4.74 19.72 3.28 18.95 4.51 13.00 1.82 11.31 1.90
T2 115.20 22.63 957.60 163.05 35.61 12.20 3.28 26.16 19.29 230.78 1.27 175.93 0.93 23.38 60.20 3.24 77.40 37.05 226.41 17.33 81.74 18.43 4.67 19.65 3.21 18.88 4.44 12.93 1.75 11.24 1.83
Oxidized T3 114.92 22.35 957.32 162.77 35.33 11.92 3.00 25.88 19.00 189.54 2.03 134.76 1.29 25.10 59.92 4.96 77.54 37.21 226.55 17.47 81.88 18.57 4.81 19.79 3.35 19.02 4.58 13.07 1.89 11.38 1.97
T4 198.04 196.47 565.44 295.89 13.45 95.04 11.12 34.00 27.13 185.53 1.14 131.70 0.94 107.72 78.00 7.58 58.15 27.37 192.96 13.92 67.44 15.72 4.09 17.34 2.89 16.29 4.16 10.82 1.64 10.39 1.63
T5 198.31 196.74 565.71 296.16 13.72 95.31 11.39 34.27 27.40 162.97 0.58 108.94 0.38 197.49 78.31 8.35 58.08 27.30 192.89 13.85 67.37 15.65 4.02 17.27 2.82 16.22 4.09 10.75 1.57 10.32 1.56
Reduced T6 198.62 197.05 566.02 291.47 14.00 95.62 11.67 34.55 27.68 79.32 0.93 25.29 0.38 197.80 78.62 8.66 58.22 27.44 193.00 13.99 67.51 15.79 4.16 17.41 2.96 16.36 4.23 10.89 1.71 10.46 1.70
S1 142.07 24.50 912.57 180.17 84.28 39.07 21.95 24.83 37.96 229.43 1.04 175.38 0.84 28.25 42.10 8.11 68.19 25.94 205.96 14.16 78.90 18.60 4.50 18.38 3.19 17.68 4.17 11.66 1.60 11.08 1.71
S2 141.62 24.05 912.12 179.72 83.83 38.62 21.50 27.38 37.51 188.19 1.80 134.21 1.20 26.80 41.62 6.66 68.26 26.01 206.00 14.23 78.97 18.67 4.57 18.45 3.26 17.75 4.24 11.73 1.67 11.15 1.78
5.0
Samples/PAAS
Formation Member Facies Sample No. Ni Co V Cr Cu Zn Mo Pb As Zr Hf Nb Ta Th U Sc Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
2.5
0.0 La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Reduced facies (Dabdab Formation)
Oxidized facies (Dabdab Formation)
Reduced facies (Tarut Formation)
Oxidized facies (Tarut Formation)
Reduced facies (Ashkidah Formation)
Oxidized facies (Ashkidah Formation)
Fig. 5: PAAS-normalized REE diagram for the oolitic ironstones
Page 42
Oxidized S3 142.40 24.83 912.90 180.50 84.61 39.40 22.28 29.16 38.29 216.12 2.61 160.93 1.27 25.58 42.40 5.44 68.12 25.87 205.89 14.09 78.83 18.53 4.43 18.31 3.12 17.61 4.10 11.59 1.53 11.00 1.64
S4 196.97 195.40 592.47 305.07 4.18 93.97 1.85 34.73 17.86 131.98 1.47 76.84 0.77 306.15 76.97 6.00 76.56 32.73 211.55 16.33 78.05 18.52 4.47 19.75 3.28 18.83 4.23 12.51 1.68 11.54 1.80
S5 197.20 195.63 592.70 300.30 4.41 94.20 2.08 33.96 18.09 98.90 0.39 43.91 0.05 306.38 77.20 6.24 76.49 32.66 211.48 16.26 77.98 18.45 4.40 19.68 3.21 18.76 4.16 12.44 1.61 11.47 1.73
S6 197.31 195.74 592.81 307.41 4.52 94.31 2.19 35.07 18.20 117.86 1.35 63.03 0.70 306.49 77.31 6.35 76.63 32.80 211.62 16.40 78.12 18.59 4.54 19.82 3.35 18.90 4.30 12.58 1.75 11.61 1.87
SGA 2017-14th Biennial Meeting of Society for Geology Applied to Mineral Deposit, Québec, Canada
Fig. 6: Ternary plots of SiO2–FeO+MgO–Fe2O3 for oolitic ironstone samples (fields after James, 1992) 2) Minette Type: This type is the most common and
the relatively high content of P2O5 (0.22-1.91), and low
widespread ironstones. The principal minerals are siderite and
TiO2/Al2O3 (0.04-0.13), may indicate a continental source for
chamosite. The Fe2O3 content is around 30%, while CaO runs
the phosphorous. Regarding the source of Fe in the studied
5-20% and SiO2 is usually above 20%. The Minette type is
oolitic ironstones, the authors also believe that Fe has been
common in the Mesozoic and Cenozoic of Europe, northern
leached from underlying sediments. This assumption is based
Africa and southern United States.
on the weak correlations between Fe2O3 and both P2O5 and Zn (r = -0.01 and -0.25, respectively). The oolitic ironstones clearly
The above statements suggest that the studied oolitic
have elevated SiO2, and Al2O3 but lesser MnO, and also lower
ironstones fall under the Clinton type.
amounts of CaO and MgO. The negative correlation of Fe2O3 with both SiO2 and Al2O3 (r = -0.9 and -0.8, respectively)
Origin
reflects the decreased deposition of detrital quartz grains and In general, there are different hypotheses regarding
fine grained detrital clay minerals during Fe-deposition, which
the source of the iron in oolitic ironstones. The Fe-enrichment
attest that iron minerals are primarily chemically precipitated.
can occur from supergene sedimentary processes (Macquaker
Si and Al data from the ironstone samples suggest a
et al., 1996) or the Fe-enrichment is hypogene including
hydrogenous origin based on their plotting in the hydrogenous
hydrothermal and/or volcanic sources (Garnit and Bouhlel,
field of the Si−Al discrimination diagram (Fig. 7).
2016).
Fe-Mn
hydrothermal,
oxyhydroxide hydrogenous,
precipitates, diagenetic
may or
be
of
mixed-type
The source of the ferromagnesian elements (Cr, Ni,
(diagenetic- hydrogenetic) origins, this terminology is based on
Co, Sc, and V) is likely to be from basic rocks; these elements
the type of aqueous fluid from which the Fe-Mn oxyhydroxides
can be supplied either from the weathering of the basic rocks
precipitate (Bau et al., 2014). High P2O5 content are recorded
present outside the basin or by within-basin volcanism (Khan
in several oolitic ironstone deposits that ranges from 0.2 to
and Naqvi, 1996). The affinity of Al, V, Cr, Mn, Mo and U for
0.8% but may sometimes exceed 1.5% (Chai et al., 2011). In
Fe-oxyhydroxides is well documented and manifests in
agreement with Baioumy et al., (2017) the authors believe that
different ways such as isomorphic substitutions or surface
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SGA 2017-14th Biennial Meeting of Society for Geology Applied to Mineral Deposit, Québec, Canada
40
Si %
30
Hydrothermal
20
Hydrogeneous
10
0 0
2
4
6
8
10
Al % Reduced facies (Dabdab Formation)
Oxidized facies (Dabdab Formation)
Reduced facies (Tarut Formation)
Oxidized facies (Tarut Formation)
Reduced facies (Ashkidah Formation)
Oxidized facies (Ashkidah Formation)
Fig. 7: Bivariate plots between Al vs. Si in the ironstone samples (fields after Choi and Hariya, 1992) adsorption (Cornell and Schwertmann, 2003). Hydrothermal
evidence of volcanic activity in the basin when these Fe-oolites
Fe-Mn deposits show higher contents of Zn, Pb, Mo, V and As
formed. Further, the absence of volcanic material in the studied
and are depleted in Co, Ni and Cu relative to hydrogenous
samples also suggests a non-volcanic origin for the Fe-oolites.
deposits (Boyd and Scott, 1999). The discrimination diagram
La/Ce ratio in the studied samples is 0.14, in average, which is
based on Ni+Co vs. As+Cu+Mo+Pb+V+Zn also indicates that
very close to hydrogenous Mn−Fe crusts (0.25, Nath et al.,
the studied ironstones display hydrogenetic type mineralization
1997).
(Fig. 8). The hydrogenous and hydrothermal deposits can be also distinguished by using Co/Ni and Co/Zn ratios (Toth,
Depositional Environment and Paleooxygenation
1980). A ratio of Co/Ni < 1 and Co/Ni > 1 indicates a sedimentary
origin
and
a
deep
marine
The depositional environment of oolites has long been
environment,
a subject of speculation and discussion. Several depositional
respectively (Oksuz, 2011). A ratio of Co/Zn of 0.15 is
environments have been proposed for oolitic ironstone: shallow
indicative of a hydrothermal type deposit and a ratio of 2.5
marine (Garnit and Bouhlel, 2016), offshore transition marine
indicates a hydrogenous type deposit (Toth, 1980). In the
(Burkhalter, 1995); restricted lagoonal marine (Bayer, 1989); or
ironstone samples, Co/Ni and Co/Zn ratios range from 0.17 to
coastal and deltaic setting environments (Collin et al., 2005).
0.98 and 0.62 to 2.12, respectively. Although Co/Zn ratios point
They are usually encountered in simply folded shallow shelf
to a hydrogenous source for Fe mineralization, Co/Ni ratios of
areas, and most typically are close to the transition from
the samples indicate that sedimentary environments played an
nonmarine to marine environments and always hosted by
important role during the formation of the Fe deposits.
clastic sediments at the top of coarsening and shoaling-upward cycles (Maynard and Van Houten, 1992).
A positive Eu anomaly and low ∑REE (2), Ni/Co (>5) and U/Th (>1.25) ratios. The
The present paper work describes the mineral and
discrimination diagrams based on SiO2 vs. Al2O3+N2O3+ K2O
chemical compositions of the oolitic ironstones at Wadi As
and CIA vs. K2O/Na2O indicated semi-humid to semi-arid
Shati, SW Libya. The three formations; Dabdab, Tarut and
conditions. Direct dating results of molybdenite from the
Ashkidah, represent the most important iron ore bearing cycles
ironstone deposits using Re-Os isotope systematics show that
of sedimentation in the Wadi As Shati area. The studied
the ages of Dabdab, Tarut and Ashkidah formations are
ironstones are divided into two facies, reduced and oxidized.
Fransnian (Late Devonian), Famennian (Late Devonian) and
The detected iron minerals are goethite, siderite, hematite,
Tournaisian (Early Carboniferous), respectively.
limonite,
magnetite,
chamosite
and pyrite.
The
oolitic
ironstones fall under the Clinton type. Regarding the source of
Acknowledgements
Fe in the ironstones, the authors believe that Fe has been
The authors thank the Thin Section Lab, Toul, France
leached from underlying sediments. This assumption is based
for thin section preparation, the ACME analytical laboratories
on the weak correlations between Fe2O3 and both P2O5 and
of Vancouver, Canada for atomic absorption spectroscopy, LOI
Zn. The discrimination diagrams based on Al vs. Si, Ni+Co vs.
and ICP-MS analyses and the Institut de Physique du Globe
As+Cu+Mo+Pb+V+Zn, ∆Ce vs. Nd and ∆Ce vs. (Y/Ho)N
de Paris, France for N-TIMS analysis.
indicate that the studied ironstones display hydrogenetic type mineralization. ΔCe values are weakly correlated with Pb, in
References
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oxidized facies display low values of V/Cr (